diff --git a/src/config.rs b/src/config.rs
index f20bbfb5..58c30dc7 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -19,12 +19,18 @@ use std::cell::LazyCell;
 use std::collections::HashMap;
 use std::fmt;
 use std::fs;
-use std::io::{BufRead, BufReader, Read, Write};
-use std::path::PathBuf;
+use std::io::{BufRead, BufReader, BufWriter, Read, Seek, SeekFrom, Write};
+use std::path::{Path, PathBuf};
 use std::str::FromStr;
 
 use configparser::ini::Ini;
 use jid::Jid;
+use nom::{
+    bytes::complete::{is_not, tag, take_until1},
+    character::complete::space0,
+    sequence::tuple,
+    IResult,
+};
 
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub(crate) enum ConfigValue {
@@ -78,6 +84,33 @@ impl fmt::Display for ConfigValue {
     }
 }
 
+struct ConfigParsed {
+    section_found: bool,
+    key_found: bool,
+    offset: u64,
+}
+
+impl fmt::Debug for ConfigParsed {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ConfigParsed")
+            .field("section_found", &self.section_found)
+            .field("key_found", &self.key_found)
+            .field("offset", &self.offset)
+            .finish()
+    }
+}
+
+fn parse_section(i: &str) -> IResult<&str, &str> {
+    let (i, (_, _, _, section, _)) =
+        tuple((space0, tag("["), space0, take_until1("]"), space0))(i)?;
+    Ok((i, section))
+}
+
+fn parse_assignment(i: &str) -> IResult<&str, &str> {
+    let (i, (_, key)) = tuple((space0, is_not("= \t\r\n")))(i)?;
+    Ok((i, key))
+}
+
 pub(crate) const DEFAULT_CONFIG: LazyCell<HashMap<&str, HashMap<&str, ConfigValue>>> =
     LazyCell::new(|| HashMap::new());
 
@@ -118,49 +151,146 @@ impl<'a> Config<'a> {
         let section: String = section
             .map(|jid| jid.to_string())
             .unwrap_or(String::from("default"));
-        self.write(key.clone(), value.clone(), section.as_str())?;
+        self.write_to_file(key.clone(), value.clone(), section.as_str())?;
         let _ = self.ini.set(section.as_str(), key, Some(value.to_string()));
         Ok(())
     }
 
+    fn parse<R: Read + BufRead>(
+        mut reader: R,
+        key: &str,
+        section: &str,
+    ) -> Result<ConfigParsed, Error> {
+        let key = key.trim().to_lowercase();
+        let section = section.trim().to_lowercase();
+
+        let mut line = String::new();
+        let mut clear_offset: usize = 0;
+        let mut sections_found: Vec<usize> = Vec::new();
+        let mut current_section: Option<String> = None;
+        let mut key_found = false;
+
+        loop {
+            let len = reader.read_line(&mut line)?;
+
+            // EOF
+            if len == 0 {
+                if current_section == Some(section.clone()) {
+                    sections_found.push(clear_offset);
+                }
+                break;
+            }
+
+            if let Ok((_, new_section)) = parse_section(&line.as_ref()) {
+                // Save offset of the section we're looking for, before replacing `current_section`
+                // value.
+                if current_section == Some(section.clone()) {
+                    sections_found.push(clear_offset);
+                }
+
+                current_section = Some(String::from(new_section));
+            } else if let Ok((_, new_key)) = parse_assignment(line.as_ref()) {
+                if current_section == Some(section.clone()) && key == new_key {
+                    sections_found.push(clear_offset);
+                    key_found = true;
+                    break;
+                }
+            }
+
+            clear_offset = clear_offset + len;
+            line = String::new();
+        }
+
+        let offset = {
+            if sections_found.len() > 0 {
+                sections_found[sections_found.len() - 1]
+            } else {
+                // If the section doesn't exist, we want to use the latest offset we've seen
+                clear_offset
+            }
+        };
+
+        Ok(ConfigParsed {
+            section_found: sections_found.len() > 0,
+            key_found,
+            offset: u64::try_from(offset)?,
+        })
+    }
+
+    fn write<R: Read + BufRead, W: Write>(
+        reader: &mut R,
+        writer: &mut W,
+        parsed: ConfigParsed,
+        key: &str,
+        value: &str,
+        section: &str,
+    ) -> Result<(), Error> {
+        let key = key.trim().to_lowercase();
+        let value = value.trim();
+        let section = section.trim().to_lowercase();
+
+        // Write up to offset value
+        let mut buffer: Vec<u8> = Vec::new();
+        buffer.resize(usize::try_from(parsed.offset)?, 0u8);
+        reader.read(&mut buffer)?;
+        writer.write(&buffer)?;
+
+        // Write new section
+        if !parsed.section_found {
+            writer.write(&format!("[{}]\n", section).as_bytes())?;
+        }
+
+        // Write new key.
+        writer.write(&format!("{} = {}\n", key, value).as_bytes())?;
+
+        // If key was found in original config, skip it
+        if parsed.key_found {
+            {
+                let mut buffer = String::new();
+                let _ = reader.read_line(&mut buffer)?;
+            }
+        }
+
+        let mut buffer = String::new();
+        reader.read_to_string(&mut buffer)?;
+        writer.write(buffer.as_bytes())?;
+
+        Ok(())
+    }
+
     /// Write config file with the least modification to the original file, and replace the
     /// original.
-    fn write(&self, key: &str, value: ConfigValue, _section: &str) -> Result<(), Error> {
+    pub(crate) fn write_to_file(
+        &self,
+        key: &str,
+        value: ConfigValue,
+        section: &str,
+    ) -> Result<(), Error> {
+        let key = key.trim().to_lowercase();
+        let value = format!("{}", value);
+        let section = section.trim().to_lowercase();
+        let tempfile = self.filename.with_extension("tmp");
+
         // Copy the file
-        let tempfile = self.filename.clone().with_extension("tmp");
-        fs::copy(self.filename.clone(), tempfile.clone())?;
-
         {
-            // Parse the file, get seek offset
-            let mut file = fs::File::open(tempfile.clone())?;
-            let mut reader = BufReader::new(file);
-            let mut line = String::new();
-            let mut clear_offset: u64 = 0;
+            let in_file = fs::File::open::<&Path>(self.filename.as_ref())?;
+            let mut reader = BufReader::new(in_file);
+            let mut out_file = fs::File::create(&tempfile)?;
+            let mut writer = BufWriter::new(&mut out_file);
 
-            let linestart = format!("{} ", key);
-            loop {
-                let len = reader.read_line(&mut line)?;
-                if line.trim().starts_with(linestart.as_str()) {
-                    // Found offset
-                    break;
-                }
-                clear_offset = clear_offset + u64::try_from(len)?;  // This can fail?
-            }
-
-            // Keep remainder of the file in memory
-            let mut remainder = String::new();
-            reader.read_to_string(&mut remainder);
-
-            let mut file = reader.into_inner();
-
-            // Clear file after the found offset. If not offset was found, clears after EOF, and
-            // the remainder is empty.
-            file.set_len(clear_offset);
-            file.write(format!("{} = {}\n", key, value).as_bytes());
-            file.write(remainder.as_bytes());
+            let parsed = Config::parse(&mut reader, section.as_str(), key.as_str())?;
+            reader.seek(SeekFrom::Start(0))?;
+            Config::write(
+                &mut reader,
+                &mut writer,
+                parsed,
+                key.as_str(),
+                value.as_str(),
+                section.as_str(),
+            )?;
         }
 
-        fs::rename(tempfile, self.filename.clone());
+        fs::rename(tempfile, self.filename.clone())?;
 
         Ok(())
     }
@@ -233,3 +363,224 @@ impl<'a> Config<'a> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::BufReader;
+
+    #[test]
+    fn parse_section_and_key_exist() {
+        let file = String::from("[section1]\n  key1= value1\n");
+        let mut reader = BufReader::new(file.as_bytes());
+        let parsed = Config::parse(&mut reader, "key1", "section1").unwrap();
+
+        assert_eq!(parsed.section_found, true);
+        assert_eq!(parsed.key_found, true);
+        assert_eq!(parsed.offset, u64::try_from("[section1]\n".len()).unwrap());
+    }
+
+    #[test]
+    fn parse_section_exists_key_doesnt() {
+        let file = String::from("[section1]\nkey1 = value1\nkey2 = value2\n");
+        let mut reader = BufReader::new(file.as_bytes());
+        let parsed = Config::parse(&mut reader, "key3", "section1").unwrap();
+
+        assert_eq!(parsed.section_found, true);
+        assert_eq!(parsed.key_found, false);
+        assert_eq!(parsed.offset, u64::try_from(file.len()).unwrap());
+    }
+
+    #[test]
+    fn parse_section_doesnt_exists_key_does() {
+        let file = String::from("[section2]\nkey1 = value1\nkey2 = value2\n");
+        let mut reader = BufReader::new(file.as_bytes());
+        let parsed = Config::parse(&mut reader, "key1", "section1").unwrap();
+
+        assert_eq!(parsed.section_found, false);
+        assert_eq!(parsed.key_found, false);
+        assert_eq!(parsed.offset, u64::try_from(file.len()).unwrap());
+    }
+
+    #[test]
+    fn parse_duplicate_section_key_exists() {
+        let file = String::from(
+            r#"
+            [section1]
+            key1 = value1
+            [section2]
+            key2 = value2
+            [section1]
+            key3 = value3
+        "#,
+        );
+        let offset = u64::try_from(
+            r#"
+            [section1]
+            key1 = value1
+            [section2]
+            key2 = value2
+            [section1]
+"#
+            .len(),
+        )
+        .unwrap();
+        let mut reader = BufReader::new(file.as_bytes());
+        let parsed = Config::parse(&mut reader, "key3", "section1").unwrap();
+
+        assert_eq!(parsed.section_found, true);
+        assert_eq!(parsed.key_found, true);
+        assert_eq!(parsed.offset, offset);
+    }
+
+    #[test]
+    fn parse_duplicate_section_key_exists2() {
+        let file = String::from(
+            r#"
+            [section1]
+            key1 = value1
+            [section1]
+            key2 = value2
+        "#,
+        );
+        let offset = u64::try_from(
+            r#"
+            [section1]
+            key1 = value1
+            [section1]
+"#
+            .len(),
+        )
+        .unwrap();
+        let mut reader = BufReader::new(file.as_bytes());
+        let parsed = Config::parse(&mut reader, "key2", "section1").unwrap();
+
+        assert_eq!(parsed.section_found, true);
+        assert_eq!(parsed.key_found, true);
+        assert_eq!(parsed.offset, offset);
+    }
+
+    #[test]
+    fn parse_offset() {
+        let file = String::from(
+            r#"
+            [section1]
+            key1 = value1
+            [section2]
+            key2 = value2
+            [section3]
+            key2 = value2
+        "#,
+        );
+        let offset = u64::try_from(
+            r#"
+            [section1]
+            key1 = value1
+"#
+            .len(),
+        )
+        .unwrap();
+        let mut reader = BufReader::new(file.as_bytes());
+        let parsed = Config::parse(&mut reader, "key3", "section1").unwrap();
+
+        assert_eq!(parsed.section_found, true);
+        assert_eq!(parsed.key_found, false);
+        assert_eq!(parsed.offset, offset);
+    }
+
+    #[test]
+    fn write_assignment() {
+        let before = String::from("[section1]\n");
+        let after = String::from("[section1]\nkey1 = value1\n");
+        let parsed = ConfigParsed {
+            section_found: true,
+            key_found: false,
+            offset: u64::try_from("[section1]\n".len()).unwrap(),
+        };
+
+        let mut reader = before.as_bytes();
+        let mut writer: Vec<u8> = vec![];
+        Config::write(
+            &mut reader,
+            &mut writer,
+            parsed,
+            "key1",
+            "value1",
+            "section1",
+        )
+        .unwrap();
+        assert_eq!(writer, after.as_bytes());
+    }
+
+    #[test]
+    fn write_assignment_remainder() {
+        let before = String::from("[section1]\n[section2]\n");
+        let after = String::from("[section1]\nkey1 = value1\n[section2]\n");
+        let parsed = ConfigParsed {
+            section_found: true,
+            key_found: false,
+            offset: u64::try_from("[section1]\n".len()).unwrap(),
+        };
+
+        let mut reader = before.as_bytes();
+        let mut writer: Vec<u8> = vec![];
+        Config::write(
+            &mut reader,
+            &mut writer,
+            parsed,
+            "key1",
+            "value1",
+            "section1",
+        )
+        .unwrap();
+        assert_eq!(writer, after.as_bytes());
+    }
+
+    #[test]
+    fn write_assignment_key_exists() {
+        let before = String::from("[section1]\nkey1 = value2\n[section3]\nkey3 = value2\n");
+        let after = String::from("[section1]\nkey1 = value1\n[section3]\nkey3 = value2\n");
+        let parsed = ConfigParsed {
+            section_found: true,
+            key_found: true,
+            offset: u64::try_from("[section1]\n".len()).unwrap(),
+        };
+
+        let mut reader = before.as_bytes();
+        let mut writer: Vec<u8> = vec![];
+        Config::write(
+            &mut reader,
+            &mut writer,
+            parsed,
+            "key1",
+            "value1",
+            "section1",
+        )
+        .unwrap();
+        assert_eq!(writer, after.as_bytes());
+    }
+
+    #[test]
+    fn write_assignment_no_section() {
+        let before = String::from("[section3]\nkey3 = value2\n");
+        let after = String::from("[section3]\nkey3 = value2\n[section1]\nkey1 = value1\n");
+        let parsed = ConfigParsed {
+            section_found: false,
+            key_found: false,
+            offset: u64::try_from("[section3]\nkey3 = value2\n".len()).unwrap(),
+        };
+
+        let mut reader = before.as_bytes();
+        let mut writer: Vec<u8> = vec![];
+        Config::write(
+            &mut reader,
+            &mut writer,
+            parsed,
+            "key1",
+            "value1",
+            "section1",
+        )
+        .unwrap();
+        assert_eq!(writer, after.as_bytes());
+    }
+}